Power supply for reducing arcing damage in glow discharge apparatus



IN GLOW DISCHARGE APPARATUS Filed Feb. 16, 1966 C K JONES ETAL POWER SUPPLY FOR REDUCING ARCING DAMAGE A ril 8, 1969 FIG.

I s1.ow TIME" TOTAL TIME CURRENT INVENTORS CLAUDE K. JONES,

STUART W. MARTIN,

THEIR ATTORNEY.

United States Patent U.S. Cl. 219121 4 Claims ABSTRACT OF THE DISCLOSURE Glow discharge voltage is periodically applied to establish a glow for time interval less than that in which arcing damage would occur on work piece and then glow is extinguished for time interval sufficient for ions to recombine should are have occurred. The glow establishing and extinguishing intervals are continuously alternated.

It is known that a work piece can be subjected to a glow discharge in a low pressure ionizable gas so as to alter the physical characteristics of the work piece due to ion bombardment. It is also known that damage can be done to the work piece rendering it unsuitable for use if the glow should break down into a local are. When an arc occurs, streamers of charged particles will attack an isolated area on the work piece and may then migrate to other areas causing severe damage.

Various proposals have been advanced for reducing the possibility of an arc, such as superimposing intensified pulses of energy on basic current level. It has been proposed that the intensified pulses be of short duration relative to the overall cycle and that the glow continue during the interval between pulses.

The problem with the foregoing arrangement is that once an arc has occurred, it will continue to attack the work piece until the voltage source is removed and the arc extinguished. Therefore, the foregoing known proposal depends upon selecting intensified pulsations of such a duration and intensity that an arc is improbable. However, this serves as a limitation upon the energy which can be applied since we have found that arcs are quite unpredictable and brief arcing can be accepted in certain circumstances so long as its time of occurrence is limited to that in which no damage to the work piece can occur.

Accordingly, one object of the present invention is to provide an improved method of reducing damage due to arcing in a glow discharge apparatus.

Another object of the invention is to provide an improved power supply for glow discharge apparatus which allows improved control over arcing.

Still another object of the invention is to provide an improved method for reducing localized dam-age to a work piece in a glow discharge device due to possible aremg.

The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in a concluding portion of the specification. The invention, however, both as to organization and method of practice, together with further objects and advantages thereof, may

3,437,784 Patented Apr. 8, 1969 "ice best be understood by reference to the following description taken in connection with the accompanying drawing in which:

FIG. 1 is a simplified schematic view of a glow discharge apparatus with associated power supply according to the preferred embodiment of the invention,

FIG. la shows the wave form of the applied voltage from the power supply,

FIG. 2 is a graph illustrating the operation of the apparatus shown in FIG. 1,

FIG. 3 is a simplified schematic view of a modified power supply, and

FIG. 3a illustrates the wave form of the modified power supply.

Briefly stated, the invention comprises periodically, first establishing a glow for a time interval which is less than that at which damage to the work piece would occur should arcing take place and, secondly extinguishing the glow for a time interval sufiicient for the ions in the arc path to recombine. When the new glow is established, it will then not be influenced by the existence of a prior arc.

Referring now to FIG. 1 of the drawing, a simplified glow discharge apparatus comprises an air tight housing 1 of electrically conductive material filled with an ionizable gas maintained at a low pressure by pump 2. A work piece 3 is disposed in container 1 and is connected to an electrode 4 which is insulated with a gas-tight insulator 5.

The power supply for the glow discharge apparatus comprises a transformer 6 having a primary coil 7 and secondary coil 8. The terminals of the latter are connected to a bridge rectifier 9 so as to provide full wave rectified voltage at terminals 10, 11. FIG. la of the drawing illustrates the wave form of the voltage appearing at terminals '10, 11. It is important to note that no filter is employed for smoothing the voltage pulses.

The positive terminal 10 of the rectifier bridge is connected via a lead 12 and a rheostat 13 to the conductive case of housing 1 which is preferably grounded for safety. The negative terminal 11 is connected via lead 14 to electrode 4.

Reference to FIG. 2 of the drawing illustrates a composite graph illustrating the operation of the device. The right hand portion shows the characteristic curve 15 of a typical glow discharge with glow current plotted as the abscissa and voltage plotted as the ordinant. The characteristic curve 15 is typical in that the applied voltage must exceed peak 15a before a glow will become established in the apparatus. A normal glow takes place along the relatively flat central portion of the curve 1512 whereas the portion illustrates the arcing region.

The operating point on characteristic curve 15 at any one time is dependent upon load lines 16, the slope of which is determined by the setting on rheostat 13.

The left hand portion of FIG. 2 illustrates the applied voltage curve 17. As the voltage rises in each half cycle toward a more positive value, the point will be reached at A where a glow is established in accordance with the intersection of the load line with the characteristic curve 15.

As applied voltage continues to rise, the glow is in operation and the operating point move-s along the flat portion 15bof the characteristic curve until point C is reached at the peak of the applied voltage curve. Thereafter, the operating point moves back along curve until at point B, where the slope of the load line is less than that of curve 15, the glow is extinguished. The process is repeated during the next period of applied voltage. By way of definition, the term period herein refers to the time between peaks of applied voltage. The term interval is defined as a portion of a period.

It will be understood that curve 17 illustrates the applied voltage and not the actual voltage drop across the glow. The latter is rather complicated and essentially consists of a sharp peak as the glow is established, dropping to a rather stable value until the time that the glow is extinguished. It again peaks an amount which varies with the amount of charges in the glow which are swept out after the glow is extinguished.

It is important to note that should an arc occur any time when the applied voltage is between points C and B on curve 17, the arc is extinguished as point B is reached, and then an entirely new glow is established as a new point A is reached on the next wave applied voltage. When the new glow is established, it has no knowledge of any are which might have occurred during the preceding period, provided a sufficient time interval is provided between that time at which the glow was extinguished and the time at which new glow is established to allow recombining of ions in the arc path in the housing 1. Therefore should a new arc occur on the next or subsequent cycles, there is little likelihood of it attacking the same local spot on the workpiece as the preceding arc.

The method by which arcing damage is reduced or eliminated is through appropriate selection of the time interval during which glow current flows and the time interval during which the glow is extinguished. These are selected respectively, in the apparatus of FIG. 1 by the frequency of the applied voltage waves shown in FIG. 1a and by the adjustment of rheostat 13 to adjust the slope of load lines 16.

By way of example, nitriding of a work piece may be accomplished in an atmosphere comprising hydrogen and nitrogen in proportions approximately the same as found in ammonia, and at a gas pressure on the order of 120 mm. Hg.

\It has been found in the aforesaid case that should an arc occur, no ascertainable damage will be done on the usual metals suitable for nitriding provided that the arc lasts for an interval less than approximately .005 second. It has also been found that should an arc occur and then be extinguished, an interval of appromixately .002 second is required for the ions to recombine to such an extent that, should a new glow be established and subsequent arcing take place, this will occur at random (with respect to the previous location attacked by the arc).

The foregoing time intervals are approximately achieved by applying a 60 cycle alternating current to the primary coil of transformer 6 and by adjusting the load line with rheostat 13, so that the glow time shown in FIG. 2 is approximately 60% of the total pulse period of .00833 second second).

It will be understood that the foregoing illustration is merely given by way of example, and that the time interval in which arcing damage would occur and the time interval required for the ions to recombine will vary with the work piece, type of gas, gas pressure, etc., and in many cases, will preferably be determined by empirical means.

Reference to FIGS. 3 and 3a of the drawing illustrates a modified form of the invention. The glow discharge apparatus, leads, and rheostat 13 are substantially the same as described previously. However, instead of a full wave rectifier power source, a synchronous rectifier is employed. This comprises a motor 19 driving slip rings 20 which are connected to the secondary of transformer 6. Rings 20 are connected to rotating commutator seg- 4 ments 21. The positive and negative leads 12, 14 are connected to segments 21 by means of brushes 22. The brushes are adjusted to be in phase with one power supply so as to obtain an interrupted wave as indicated in FIG. 3a.

It will be observed that the time interval between pulses and the time duration of the glow are adjusted either by varying the space between commutator segments 21, or preferably by adjustment of rheostat 13 as before. In the latter case, the interval between segments 21 occupies about 20% of the total period. If the rheostat is adjusted so that the glow is off for 40% of the total period, then polarity reversal takes place when no current is flowing.

The use of a synchronous rectifier as disclosed is preferable for the above reason in some cases. This is because solid state rectifiers shown in FIG. 1 are often sensitive to high voltage spikes of relatively short duration. In the synchronous rectifier employed in FIG. 3, since the glow is extinguished and no current is flowing when polarity reversal occurs, arcing at the segments when switching takes place is largely avoided. This makes the synchronous rectifier ideal for some applications as a substitute for the simpler arrangement shown in FIG. 1.

Thus it will be seen that selection of the time intervals both of applied glow energy and of glow extinguishment can be selected to apply the maximum energy to the sample without damage. It is not intended to infer herein that conditions are selected so that arc will most certainly take place during each interval of applied voltage. It is know-n that arcs are often random in occurrence and quite unpredictable. In fulfilling the object of the present invention, conditions are selected so that glow discharge processes can be conducted with conditions of a relatively low probability of arcing. However, the conditions can be selected which result in higher energy transformation than if they were limited by conservative values used in the past, particularly during initial heating up of the workpiece.

Further objects and advantages of the invention will occur to those skilled in the art, and it is intended to cover in the appended claims all such modifications as fall within the true spirit and scope of the invention.

What is claimed as new and desired to obtain by Letters Patent of the United States is:

1. The method of reducing damage which might occur due to arcing in a glow discharge apparatus where a workpiece disposed in a low pressure ionizable gas is subjected to a glow discharge by application of electric potential from a voltage source, said method comprising:

applying DC voltage from said source to establish a glow for a first time interval less than approximately .005 second which is selected to be less than that at which damage to the workpiece would occur should arcing take place during the first time interval, and

extinguishing the glow for a second time interval of at least .002 second which is selected to be sufficient for the gas to substantially deionize by recombination of the ions in the arc path should an are have occurred during the first time interval, and

alternately establishing and extinguishing said glow for said first and second time intervals respectively in a uniformly repetitive manner.

2. The method according to claim 1 wherein said first and second time intervals are obtained by applying a full wave rectified A-C voltage at a frequency selected to provide D-C pulses with a period equal to the sum of said first and second time intervals and by adjusting the relative length between the first and second time intervals by means of adjusting a variable resistance in the glow discharge circuit.

3. The method according to claim 1 and employing a synchronous rectifier as the voltage source and varying a resistance in series with said source to provide separated D-C pulses of applied voltage to the glow discharge apparatus with a pulse period equal to the sum of the first and second time intervals and of a duration greater than 3,004,133 10/1961 Berghaus et a1 219--121 that of the first time interval, whereby no current flows as 3,035,205 5/1962 Berghaus et a1. polarity reversal in the synchronous rectifier takes place. 3,181,029 4/ 1965 Berghaus.

4. The method according to claim 1 wherein the first 3,190,772 6/1965 Berghaus et al. interval and the second interval are on the order of 60% r 3, 2 1/1966 Berghaus e131 and 40% of a total second period respectively. 3,282,814 11/ 1966 Berghaus 204-177 References Cited RICHARD M. WOOD, Primary Examiner. UNITED STATES PATENTS W. D. BROOKS, Assistant Examiner.

2,884,511 4/1959 Berghaus et a1 219-121 10 US. Cl. X.R.

2,921,178 1/1960 Bucek 219-121 219-131 

